make FeatureFiltererMixin more powerful, add ability to remove individual filters from the filter chain
add support for supplementary_alignment flag to BAM features, make qc_failed, duplicate, and secondary_alignment flags visible in tags() on non-multi-segment reads
Added options to Alignments2 tracks to allow hiding reads that are
duplicates, fail vendor QC, have missing mate pairs, are secondary
alignments, and/or are supplementary alignments.  References #332.